1. Field of the Invention
The present invention generally relates to the field of pipe connections and to devices used in the pipeline construction industry. More particularly, this invention relates to devices used to join the ends of a polyolefin pipe, such as a polyethylene pipe, to a pipe of a different material such as PVC or ductile iron.
2. Description of the Prior Art
Typical water pipe systems use rigid pipes such as ductile iron, cast iron, or concrete which mate with fittings such as mechanical joints. While such pipes are advantageously strong, many are very heavy and cumbersome to work with, and in the case of iron suffer from corrosion. In recent years, it has been the practice of many contractors to employ polyolefins such as polyethylene, particularly high density polyethylene (HDPE), and plastics such as polyvinyl chloride (PVC) in plastic pipe used in new construction. However, many of the underground sewer and water distribution pipes, fittings and valves still contain cast iron and ductile iron. Since these dissimilar materials cannot be joined by conventional means, such as brazing, soldering or gluing, so called “mechanical joint” connections and gland restraining devices have been adapted to provide a fluid-tight connection.
Some current gland restrainer systems designed for PVC piping employ mechanical restrainer connections intermittently disposed around the PVC pipe in a gripping fashion. The gland segments of these devices are then bolted together to complete the connection. As a typical example, one system which exists in the market place today for PVC plastic pipe connections is the “CERTA-LOK”™ system sold by CertainTeed Corporation, of Valley Forge, Pa. This system provides a restrained joint between PVC pipe for municipal, fire protection, and other uses. This system offers certain advantages, since the restraining mechanism is uniformly distributed around the PVC pipe. As a result, the risk of damaging the plastic side walls by localized stress fracture is minimized. However, there has been no similar technique for joining sections of pipe of dissimilar materials together where the pipe in question are polyolefins such as polyethylene, HDPE.
In some situations, HDPE offers advantages over the use of PVC as a pipe material. For example, the PVC pipe may crack when flexed. The flexible polyolefin-type plastic pipes, including HDPE, are lightweight, easy to work with, corrosion resistant and can be fused together at the joint to form a continuous pipe to thereby minimize leakage. Polyolefin pipe, however, also suffers from certain drawbacks that have impeded its widespread use in water pipe systems and similar fluid transport systems.
As in the case of PVC pipe, the task of mechanically mating the end of a polyolefin pipe, such as HDPE, to some of the other members in a water system, and particularly to metal mechanical joints, presents significant problems. This is due, in part, to the cold flow properties of the polyolefin-type plastic. In some cases, the ends of such pipes tend to deform under pressure leading to an inadequate seal at the mechanical connection. Moreover, the pipe ends may work loose from the mechanical joint due, for example, to the greater expansion/contraction rate of that type of plastic as compared to other more rigid pipes. In such a case, the connection pulls apart.
One attempted solution to the above noted problems has been to stiffen the pipe end so that it will neither deform under pressure nor work lose from the joint. One approach to solving this problem involves force fitting a rigid tube such as a steel tube inside the polyolefin pipe at the pipe end in sealing engagement with the inner diameter of the plastic pipe wall. The rigid tube act as a stiffener at the pipe end. And while the stiffening tube is believed to provide better resistance to pipe end deformation, the polyolefin pipe end may still come loose in use. Other attempted solutions to the problems of joining polyolefin pipe to pipes of different materials have involved complicated mechanical arrangements with a number of parts which were often difficult and cumbersome to install.
A need exists therefore, for an improved system for mechanically coupling polyolefin water pipes to pipes of different material such as cast iron or ductile iron pipes.
A need also exists for a female or bell mechanical joint adapter for permitting the assembly of fluid-type fittings containing multiple materials, such as HDPE, PVC and cast and ductile iron.
There also remains a need for providing an inexpensive mechanical bell joint adapter for joining HDPE to ductile or cast iron or PVC which can be field-assembled with little chance of error and with a minimum amount of assembly time.